1. Field of the Invention
The present invention relates to a method and an apparatus for controlling an air-fuel ratio, utilizing an air-fuel ratio detection result of a so-called wide range air-fuel ratio sensor.
2. Related Art of the Invention
There have been known so-called wide range air-fuel ratio sensors such as disclosed in Japanese Unexamined Patent Publication Nos.1-123141 and 1-124758.
These sensors are adapted to detect a concentration of particular component (such as oxygen) in exhaust gas, and based thereon, the air-fuel ratio of engine is detected over a wide range (in both of lean and rich air-fuel ratio ranges).
However, there exists such a problematic possibility to be noted later, just because the wide range air-fuel ratio sensors are adapted to linearly detect an air-fuel ratio over a wide range from rich to lean.
To be noted before explaining the problematic possibility, it is impossible to accurately seize a deviation amount itself of an actual air-fuel ratio from a theoretical air-fuel ratio, in case that a feedback control of air-fuel ratio is executed based on a proportional-plus-integral control making use of output value from an oxygen sensor which outputs a lean/rich inversion signal for a theoretical air-fuel ratio. Thus, the object (fuel injection quantity or intake air quantity) of the air-fuel ratio control is increased or decreased until the output value of the oxygen sensor is rich/lean inversed at the next time. As a result, this situation is repeated such that the object of the air-fuel ratio control is decreased or increased until the output value of the oxygen sensor is again lean/rich inversed at the next time, once the output value of oxygen sensor has been again rich/lean inversed. The control is performed in such a manner as noted above, so that the actual air-fuel ratio is oscillated or reciprocated at a predetermined period with a relatively large amplitude, about the theoretical air-fuel ratio (i.e., rich/lean inversed at a predetermined period).
With respect now to the problematic possibility, in case that the feedback control of air-fuel ratio is performed making use of a wide range air-fuel ratio sensor which can linearly detect air-fuel ratio over a wide range, from rich to lean, the deviation amount itself of an actual air-fuel ratio from a theoretical air-fuel ratio can be detected even if the actual air-fuel ratio has somewhat deviated from the theoretical air-fuel ratio. As such, the target (fuel injection quantity or intake air quantity) of the air-fuel ratio control is increased or decreased to such an extent corresponding to the deviation amount, to thereby correct or compensate the deviation. Thus, the oscillation amplitude of the actual air-fuel ratio to rich/lean range about the theoretical air-fuel ratio does not become so large as in case of the oxygen sensor.
Just as such, the opportunities or occasions of the rich/lean inversion in the conventional feedback control making use of the wide range air-fuel ratio sensor are decreased for the air-fuel ratio of exhaust gas at an inlet portion of an exhaust gas purification catalytic converter, as compared to the feedback control of air-fuel ratio making use of an oxygen sensor. Then arises such a possibility that the adsorption and desorption of oxygen molecules onto and from the catalytic converter surface are not effectively caused, so that the efficiency for simultaneously purifying the three components (NO.sub.x, CO, and HC) may be deteriorated.